Insects have a fat body. It’s a versatile organ, a sort of combination of adipose tissue (the blubber that humans have) and liver. Its principal roles are:
- As with adipose tissue, it is the main nutrient storage site (Telfer & Kunkel, 1991);
- Storage of neutralised waste metabolites;
- Detoxification of ammonia from protein metabolism (Scaraffia et al., 2010);
- Control of nutrient levels in the haemolymph (“bloodstream”) (Colombani et al., 2003);
- An immunity organ, producing antimicrobial peptides in response to bacterial or fungal intrusion into the haemolymph (Lemaitre & Hoffmann, 2007);
- In females, producing vitellogenin, the precursor to egg yolk and thus critical for reproduction (Roy & Raikhel, 2011).
The fat body is composed mainly of lipid-, protein-, and glycogen-rich cells called trophocytes. As the individual insect goes through more life stages, the fat body grows as the trophocytes become larger and develop vacuoles in which the fat, glycogen, and proteins are kept to serve as nutrient stores, the energy from which is used mostly for reproduction and locomotion (Arrese & Soulages, 2010). This is the closest thing to body fat that you will have in insects.
Another important cell type found in the fat body is the oenocyte, although it can also be found in association with the epidermis. Oenocytes are hydrocarbon factories, play a role in maintaining homeostasis, and in detoxification (Martins et al., 2011).
The fat body is found beneath the epidermis (like adipose tissue) and, in some insects, around the alimentary canal and testes (Roma et al., 2010). The trophocytes are linked by a thin lamina that forms lobes and ribbons into the haemocoel, increasing the fat body’s surface area and thus allowing more nutrient exchange between the fat body and haemolymph (Arrese & Soulages, 2010).
The differentiation of the fat body begins fairly early in development during the embryonic stage. In all insects, the trophocytes grow as more food is ingested. In holometabolous insects, those that undergo a metamorphosis, the larva is specifically a feeding stage during which it must accumulate as many energy stores as possible to be able to metamorphose (Aguila et al., 2007). These stores are released during the prepupal stage to be used up during pupation, when metamorphosis takes place. During metamorphosis, the highly-organised larval fat body is restructured into a loose mass of connected trophocytes (Nelliot et al., 2006). The larval trophocytes become disconnected and form clumps in the pupal body. These trophocytes then undergo programmed cell death in the new adult, and are replaced in the adult by new trophocytes derived from the body wall (Hoshizaki et al., 1995). The larval trophocytes do remain in the adult as energy reserves until a feeding site is found (Aguila et al., 2007).
Arrese EL & Soulages JL. 2010. Insect Fat Body: Energy, Metabolism, and Regulation. Annual Review of Entomology 55, 207-225.
Martins GF, Ramalho-Ortigão JM, Lobo NF, Severson DW, McDowell MA & Pimenta PFP. 2011. Insights into the transcriptome of oenocytes from Aedes aegypti pupae. Memórias do Instituto Oswaldo Cruz 106, 308-315.
Roy SG & Raikhel AS. 2011. The small GTPase Rheb is a key component linking amino acid signaling and TOR in the nutritional pathway that controls mosquito egg development. Insect Biochemistry and Molecular Biology 41, 62-69.
Scaraffia PY, Zhang Q, Thorson K, Wysocki VH & Miesfeld RL. 2010. Differential ammonia metabolism in Aedes aegypti fat body and midgut tissues. Journal of Insect Physiology 56, 1040-1049.
Telfer WH & Kunkel JG. 1991. The Function and Evolution of Insect Storage Hexamers. Annual Review of Entomology 36, 205-228.